The present invention relates to a switch arrangement and, more particularly, to an electronic proximity switch dependent upon a magnetic field operated by an approaching magnetic trigger, with an HF tank circuit of an oscillator circuit which can be acted upon by the magnetic trigger, and with a coil arrangement of the oscillator circuit, including a tank circuit coil and coupling coil, being associated with a magnetic member which latter, starting with a specific magnetic field strength, can be driven into magnetic saturation by the external magnetic field of the magnetic trigger with undamping of the tank circuit.
In, for example, Auslegeschrift No. 1 175 327, a proximity switch is proposed which includes an oscillator circuit wherein a permeable metal plate is disposed between the tank circuit coil and the feedback coil, with the metal plate extensively shielding the tank circuit coil with respect to the coupling coil so that only greatly damped oscillations occur. By the approach of the permanent-magnet trigger, the metal plate in this arrangement can be driven into saturation whereby the shielding effect of the plate is reduced in a step- or jump-like fashion and, consequently, a marked undamping of the tank circuit results. This effect is exploited for operating an electronic switch.
Since the tank circuit, in the basic condition, is greatly damped, that is, does not transmit an output signal, flaws in the electronic circuit, leading in all cases to a loss of signal but not to excitation, cannot erroneously be transformed into a switching signal. However, the conventional proximity switch shows low response sensitivity for its structural size and, for this reason, it cannot be triggered when fashioned of a compact size, for example, with a space requirement of merely 1 cubic centimeter, through impermeable metallic walls of, for example, aluminum or austenitic steel.
The aim underlying the present invention essentially resides in providing a proximity switch of the aforementioned type having a small structural size and a high response sensitivity.
In accordance with the present invention, a magnetizable member includes an amorphous or predominantly amorphous metal band, with the tank circuit coil and coupling coil being seated on a joint bobbin. Amorphous metal, also called metallic glass in the United States and sold in Germany under the trademark "VITROVAC", is a material exhibiting high permeability but having an electrical conductivity which is low as compared with transformer plates or dynamo sheets.
In accordance with the present invention, the tank circuit, in the basic condition, is greatly damped and is suddenly excited to strong oscillations by an approaching magnetic trigger, resulting, even with a small structural size, in a high response sensitivity. Since the tank circuit coil and the coupling coil are arranged on a joint bobbin, the magnetic trigger, however, affects the coupling characteristics between the two coils either not at all or only to a negligible extent. The decisive factor, in spite of the relatively low electrical conductivity of the amorphous metal band, is the induction of very strong eddy currents in the amorphous metal band. The metal band in the basic condition, appears to bind the HF tank circuit field so closely to itself, due to the high permeability thereof that large eddy current losses occur.
Upon the approach of the magnetic trigger, the magnetic field of the latter is likewise attracted in a special manner by the amorphous metal band so that this field is quickly driven into saturation. The strong undamping of the tank circuit observed under practical conditions, however, is surprising since it is to be expected that, upon a decrease of the permeability of the amorphous metal band, the inductance of the tank circuit would decrease and, thus, the resonant frequency would rise and, consequently, there should be actually an increase in the eddy current losses.
In any event, in practice, a very pronounced changeover of the damping condition can be observed. Additional advantages in using an amorphous metal band are good viscoelastic properties rendering the proximity switch insensitive, in particular, against vibrations and shocks, as well as the fact that the characteristic values are independent from temperature fluctuations, thereby permitting use of a sensor head in a temperature range of between -50.degree. C. and +150.degree. C. without temperature compensation.
In accordance with advantageous features of the present invention, the amorphous metal band can be fashioned as a straight thin strip arranged along an axis of the coil arrangement and passing through a ring or torus of the coil thereby making it possible to obtain very high response sensitivities, increasing with an increasing length of the amorphous metal band. Even at a band length of about 1 centimeter, response distances can be attained of about 10 centimeters.
Preferably, the proximity switch of the present invention is operated by a trigger moving along the proximity switch, the amorphous metal band can have the shape of an inherently or substantially closed ring or oval, with the ring or oval, respectively, passing through the coil axis and encompassing the coil ring or torus. In conventional proximity switches such as, for example, reed contacts, there is the problem of multiple triggering during movement of the trigger along the proximity switch. By fashioning the amorphous metal band in the form of an inherently or substantially closed ring or oval, only a single switching step occurs, in contrast to the conventional proximity switch, while the magnetic trigger is moved along the arrangement. Additionally, with such a transverse movement of the magnetic trigger, the switching zone is narrowly limited so that a very accurate positional detection of the magnetic trigger is ensured. Furthermore, by a simple deformation of the annular metal band, the characteristic data of the proximity switch can be influenced within certain limits. In this connection, it is advantageous that the amorphous metal band does not incur any change in its magnetic property by bending or similar deformations.
In accordance with further advantageous features of the present invention, it is possible to provide two or more inherently closed annular or oval metal bands distributed along the ring or torus of the coil thereby making it possible to affect the characteristic data of a proximity switch. A particular advantage resides in an enlargement of the response range upon movement of the magnetic trigger toward the proximity switch, whereas, the response zone remains still very narrow with respect to a transverse movement of the trigger.
According to the present invention, the metal band may have a thickness of between 15 .mu.m and 50 .mu.m, preferably being about 30 .mu.m, and a width of 1 to 3 millimeters. Furthermore, according to the present invention, a diameter of the ring may be on the order of 2 to 10 millimeters. In spite of the small structural sizes, the proximity switch of the present invention readily attains very high response sensitivities.
Advantageously, according to the present invention, the amorphous metal band is fashioned as a U- or C-shaped arc section, with an opening thereof lying on a response side of the proximity switch. Preferably, a very narrow far-reaching parabola-shaped response zone is created thereby especially avoiding the creation of secondary response zones. Such a proximity switch is readily employable for a great variety of different sensitivity ranges and makes it unnecessary to inventory or warehouse various types of proximity switches. A crest of an arcuate section of the substantially U- or substantially C-shaped arc sections is, advantageously located in the coil ring or torus whereby an exactly symmetrical response zone is produced.
According to further features of the present invention, the width of the opening of the arc section can be approximately the same as or smaller than an axial dimension of the coil ring or torus. An embodiment especially favorable from a manufacturing viewpoint results with an annular configuration of the arc section and a ring having an opening width which is approximately equal to or smaller than a radius of the arc section. With a radius of about 3 millimeters for the annular arc section, the opening width is preferably about 2.5 millimeters.
Alternatively, according to the present invention, the crest of the arc section lies on the end face of the coil ring or torus opposite to the response side, and the arc section embraces, with lateral legs thereof, half of the coil ring or torus.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purpose of illustration only, several embodiments in accordance with the present invention.